A significant percentage of mining operations utilize hydrometallurgical treatments for processing metal-bearing materials such as ores, concentrates, and the like. Such processes typically involve the liberation of metal ions from the metal-bearing material using a leaching step in which the metal-bearing material is treated with an acid solution, either atmospherically or under elevated temperature and pressure conditions. As a result of the leaching process, metal ions are transferred to the acid thereby yielding a pregnant leach solution having an increased metal ion concentration. The pregnant leach solution may thereafter be subjected directly to a process such as cementation or the like to extract the metal content. Alternatively, the pregnant leach solution may be subjected to a solvent extraction process in which an organic solvent or extractant removes the metal from the pregnant leach solution. The loaded extractant is then mixed with an aqueous acid solution which strips the metal from the extractant thereby producing a solution stream with high metal concentration suitable for treatment in an electrowinning circuit or other subsequent treatment as may be desired to extract the concentrated and relatively pure metal. Of course, these general treatment procedures may be modified as desired depending upon the particular metal being extracted.
The hydrometallurgical processes which are typically utilized rely on the substantially continuous availability of sulfuric acid solution for use in the leaching step as well as in stripping the metal from any organic extractant as may be utilized. Such processes may also require the availability of acid to make up for losses in tank house solutions used in electrowinning circuits. The desired concentration levels for acid solutions used in these various processes is typically in the range of about 10 grams per liter acid to about 50% acid by weight. As will be appreciated, the various hydrometallurgical processes circulate large quantities of diluted acid solutions as a result of the various processing steps. These by-product acid streams typically have relatively dilute acid concentrations. Thus, hydrometallurgical processes used in metal extraction require the constant availability of fresh acid feedstock.
Historically, sulfuric acid solutions used in hydrometallurgical processing have been produced in acid plants providing acid at concentrations in the range of about 93% to about 98% acid. At lower acid concentrations (and at typical acid plant operating temperatures) the acid may become highly corrosive to metals, thereby giving rise to equipment and piping degradation. Due to the high concentration of the acid being produced, current acid plants are designed to operate under closely controlled temperature conditions. Moreover, such traditional acid plants must also operate substantially without the presence of moisture to avoid the generation of undesirable acid mists. Such stringent design criteria may give rise to substantial complexity in the design and operation of traditional acid plants.
As noted previously, in hydrometallurgical processing, it is often desirable to utilize relatively low concentration acid streams in the range of about 10 grams per liter acid to about 50% acid by weight. Within these ranges, the acid solutions provide effective leaching and solvent stripping characteristics while nonetheless being relatively non-corroding. In order to reach these desired concentrations, acids produced by typical prior processes are generally intentionally diluted by adding the acid to water.